Dear ixtab,
I did anything as you asked. I installed suitable Jailbreak and Font Hack too. My Kindle's fw is 5.1.2.
However Kindle still ask me the Jailbreak. What will I do? Please help me! Thanks a lot.

Dear ixtab,
I did anything as you asked. I installed suitable Jailbreak and Font Hack too. My Kindle's fw is 5.1.2.
However Kindle still ask me the Jailbreak. What will I do? Please help me! Thanks a lot.

Well, you obviously didn't do everything that is required. Did you read the README? Did you follow the instructions? If not, did you wonder why the file is called README?

I don't know whether restart of whole framework is needed to handle this file (or it's abscence) or just restart of reader booklet is sufficient, but believe former is true.

Wow! This font hack even works on the new Kindle Paperwhite with firmware 5.3.0.
1.) just plug in the Kindle to your OS
2.) copy an empty file with name USE_ALT_FONTS to root directory of kindle (internally the root device is /mnt/us/)
3.) create an folder "fonts" (/mnt/us/fonts)
4.) copy your fonts to this folder (4 files for each font: Fontname-Regular.ttf, Fontname-Italic.ttf, Fontname-Bold.ttf and Fontname-BoldItalic.ttf)
5.) restart your Kindle and from now on you will see in the "Aa" menu of the standard reader app your own fonts as well

P.S. My Paperwhite is already jailbroken and so I don't know if the jailbreak is necessary for the font hack or not.